The present invention generally relates to seals for steam turbines and particularly to packing casing seals at the horizontal midline joint for sealing between high and intermediate pressure regions of high and intermediate pressure turbine sections and methods of forming the seals.
Steam turbines are oftentimes comprised of axially spaced high and intermediate pressure turbine sections about a rotor common to each section. As well known, each section includes a plurality of circumferentially spaced buckets mounted on the rotor at axially spaced positions forming with nozzle assemblies attached to an outer shell, a plurality of turbine stages. Between the high pressure and intermediate pressure sections of the steam turbine, there is a packing casing. The packing casing is formed typically of a plurality of shells, i.e., conventionally two half-shells, joined to one another along a horizontal midline. The packing casing includes a plurality of axially spaced arcuately extending grooves for receiving packing seal segments mounting labyrinth teeth for sealing about the rotor. The packing casing extends from the rotor seals generally radially outwardly to a connection with the outer shell of the turbine. The packing casing therefore lies intermediate the steam inlets for the high and intermediate pressure turbine sections. Because the packing casing separates the high pressure inlet flow and the intermediate pressure inlet flow, there is substantial potential for significant leakages of steam across the joint between the shells of the packing casing. Particularly, while the shells are bolted together, thermal variation through the shell during turbine operation, machining tolerances and pressure distortion afford potential for formation of leakage paths through the horizontal joint between the high and intermediate pressure regions. While spring-loaded keys have previously been used in the horizontal joint, those keys do not perform a substantial sealing function. It will be appreciated that significant steam leakage flows past the horizontal joint causes lower machine efficiency and performance.
In accordance with a preferred embodiment of the present invention, there is provided a spline seal at the joint interface between the packing casing shells to minimize or eliminate steam leakage paths between the high pressure and intermediate pressure regions of a steam turbine having high pressure and intermediate pressure sections. In each of the endfaces at the joint between adjacent packing casing shells, there is provided a groove opening in a circumferential direction and in registration with a corresponding groove in the adjacent endface. A spline seal is disposed in the registering grooves. The grooves and, hence, the spline seals extend from a location adjacent the rotor to a location adjacent the connection between the packing casing and the outer shell.
More particularly, the connection between the packing casing and the outer shell includes axially butting axial load surfaces whereby the axial load surface on the outer shell serves as a stop, preventing axial movement of the packing casing toward the intermediate pressure section. Additionally, the packing seal segments carried by the packing casing also have cooperating axial load surfaces serving as seals. The groove and spline seals extend from their radially innermost ends adjacent the cooperable axial load surfaces of the packing casing and packing seal segments to adjacent the engaging axial load surfaces between the outer shell and the packing casing. It will be appreciated that the spline seals in the horizontal joint of the packing casing can be applied to original equipment manufactures, as well as retrofitted to existing steam turbine units to increase total machine efficiency.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising axially spaced turbine sections about a common rotor operable at different pressures, an outer shell about the differential pressure sections and the rotor, a packing casing intermediate the differential pressure sections and connected to the shell, the packing casing separating differential pressure regions on axially opposite sides of the packing casing, the packing casing being formed of at least a pair of arcuate shells having circumferentially registering endfaces joined one to the other forming at least one joint therebetween and between the shell and the rotor, registering grooves in the joint endfaces and a spline seal extending in the registering grooves and sealing between the differential pressure regions on opposite axial sides of the packing casing at the joint.
In a further preferred embodiment according to the present invention, there is provided a steam turbine comprising axially spaced high and intermediate pressure turbine sections about a common rotor, an outer shell about the high and intermediate pressure sections and the rotor, a packing casing intermediate the high and intermediate pressure sections separating high and intermediate pressure regions on axially opposite sides of the packing casing, the packing casing having an axial load surface bearing against an axial load surface carried by the shell for precluding axial movement of the packing casing relative to the shell in response to the pressure differential between the high and intermediate pressure regions, the packing casing being formed of a pair of arcuate shells having circumferentially registering endfaces joined to one another forming a pair of joints therebetween and extending between the shell and the rotor substantially adjacent a horizontal midline of the turbine, registering grooves in each of the endfaces of the packing casing at each joint thereof and spline seals in the registering grooves in each joint sealing between the high and intermediate pressure regions and extending in a generally radial direction from adjacent the rotor to adjacent the axial load surfaces of the packing casing and the shell.
In a further preferred embodiment according to the present invention, there is provided in a steam turbine having axially spaced high and intermediate pressure sections about a common rotor, an outer shell about the high and intermediate pressure sections and the rotor, a packing casing intermediate the high and intermediate pressure sections and separating high and intermediate pressure regions on axially opposite sides of the packing casing, the packing casing being formed of a pair of arcuate shells having circumferentially registering endfaces joined to one another forming a pair of joints therebetween and extending between the rotor and the outer shell substantially adjacent a horizontal midline of the turbine, a method of sealing between the high and intermediate pressure regions on opposite sides of the packing casing, comprising the steps of forming a groove in each endface of the packing casing shells for registration with the groove formed in the opposed endface, inserting a spline seal into the grooves of a pair of endfaces and securing the packing casing shells to one another along the horizontal midline with the spline seals in the registering grooves of the joints.